1. Field of the Invention
The present invention relates to a novel sulfonate advantageously usable as a photoacid generator and the like in a resist composition, a polymer using the same, a method for producing the polymer, a resist composition, and a patterning process.
2. Description of the Related Art
As LSI progresses toward a higher integration and a further acceleration in speed, a finer pattern rule is being requested. In such a movement, a deep-ultraviolet lithography is drawing an attention as the promising next-generation fine processing technology.
In recent years, technologies utilizing a KrF excimer laser having a high brightness and an ArF excimer laser having a further shorter wavelength have been drawing an attention as the light sources of a deep-ultraviolet ray. In addition, an ArF immersion lithography, which can be designed to have 1.0 or more of the numerical aperture (NA) of a projection lens by inserting a liquid having a higher refractive index than that of an air, such as water, ethylene glycol, and glycerin between the projection lens and a wafer, thereby attaining a high resolution, is rapidly drawing a growing attention (see, for example, Journal of Photopolymer Science and Technology Vol. 17, No. 4, p 587 (2004)). A further fine processing technology is sought due to shifting toward a shorter wavelength of the exposure light and attaining a higher resolution in the resist composition.
From this view point, a chemically amplified resist composition catalyzed by an acid, which has been developed recently, has excellent properties in sensitivity, resolution, and dry-etching resistance, and thus is a promising resist composition, particularly for a deep-ultraviolet lithography. In this chemically amplified resist composition, there are a positive type in which an exposed area is removed with leaving an unexposed area unremoved and a negative type in which an unexposed area is removed with leaving an exposed area unremoved.
In a chemically amplified positive resist composition using an alkaline developer, a resin and/or a compound whose part or all of alkaline-soluble phenol group or carboxylic acid group is protected by an acid-unstable protection group (an acid-labile group) is catalytically decomposed, by an acid generated by an exposure, to generate a phenol or a carboxylic acid in the exposed area, thereby removing this exposed area by an alkaline developer. On the other hand, in a chemically amplified negative resist composition, a resin and/or a compound having an alkaline-soluble phenol or carboxylic acid is crosslinked, by an acid generated by an exposure, with a compound (acid-crosslinker) that can link (crosslink) the resin or the compound by the acid to insolubilize the exposed part in an alkaline developer, thereby removing the unexposed part by the alkaline developer.
In the chemically amplified positive resist composition, a base resin having the acid-labile group and a compound generating the acid by radiation irradiation (hereinafter referred to as an photoacid generator for short) are dissolved in a solvent, and the resist solution thus prepared is applied on a substrate by various ways, heated if necessary and then the solvent is removed, to form a resist film. Subsequently, the formed resist film is exposed to a light source such as a deep-ultraviolet ray by radiation irradiation through a prescribed mask pattern. Further, as appropriate, a post exposure bake (PEB) is done after the exposure to carry out an acid-catalyzed reaction, and the development by an alkaline solution is done to remove the exposed area of the resist film to obtain a positive pattern profile. After the substrate is etched by various ways, the remaining resist film is removed by dissolving in a stripping solution or by asking to form a pattern profile on the substrate.
The ArF lithography started partial use from the fabrication of 130-nm node devices and became the main lithography since 90-nm node devices. Although lithography using F2 laser (157 nm) was initially thought promising as the next lithography for 45-nm node devices, its development was retarded by several problems. An ArF immersion lithography, which can be designed to have 1.0 or more of the numerical aperture (NA) of a projection lens by inserting a liquid having a higher refractive index than that of an air, such as water, ethylene glycol, and glycerin between the projection lens and a wafer, thereby attaining a high resolution, is rapidly drawing a growing attention (see, for example, Journal of Photopolymer Science and Technology Vol. 17, No. 4, p 587 (2004)).
In the ArF lithography, a high sensitivity resist composition capable of achieving a high resolution at a small dose of exposure is required to prevent deterioration of precise and expensive optical system materials. Among several measures for realizing such a composition, the most common is to select each component which is highly transparent at the wavelength of 193 nm. For example, polyacrylic acid and derivatives thereof, norbornene-maleic anhydride alternating copolymers, polynorbornene, ring-opening metathesis polymers, hydrogenated ring-opening metathesis polymers and the like have been proposed as the base resin. This choice is effective to some extent in that the transparency of a resin alone is increased.
Studies have also been made on photoacid generators. In this case, when photoacid generators capable of generating alkane- or arene-sulfonic acid as used for conventional chemically amplified resist compositions for lithography using KrF excimer laser as a light source are used for a component of the above ArF chemically amplified resist compositions, it has been found that an acid strength to scissor acid labile groups on the resin is insufficient, resolution cannot be performed at all or sensitivity is low, and thus, these photoacid generators are not suited for the fabrication of devices.
For the above reason, photoacid generators capable of generating perfluoroalkanesulfonic acid having a high acid strength are generally used in ArF chemically amplified resist compositions. These photoacid generators capable of generating perfluoroalkanesulfonic acid have already been developed for use in the KrF resist compositions. For example, Japanese Patent Laid-Open (kokai) No. H11-282168 describes photoacid generators capable of generating perfluorohexanesulfonic acid, perfluorooctanesulfonic acid, perfluoro-4-ethylcyclohexanesulfonic acid, and perfluorobutanesulfonic acid. Japanese Patent Laid-Open (kokai) No. 2002-214774 describes novel photoacid generators capable of generating perfluoroalkyl ether sulfonic acids.
Among these compositions described above, perfluorooctanesulfonic acid derivatives (PFOS) have environmental problems such as nondegradability and concentration in the environment, so that manufacturers have made efforts to develop partially fluorinated alkane sulfonic acids having a reduced degree of fluorine substitution for alternatives. For example, Japanese Patent Application Publication No. 2004-531749 discloses a salt of α,α-difluoroalkanesulfonic acid, which is developed by using α,α-difluoroalkene and a sulfur compound, and resist compositions including a photoacid generator capable of generating the sulfonic acid by exposure, specifically, di(4-tert-butylphenyl)iodonium 1,1-difluoro-2-(1-naphthyl)ethanesulfonate, and furthermore, above-described Japanese Patent Laid-Open (kokai) No. 2002-214774 discloses difluorosulfoacetic acid alkyl esters, difluorosulfoacetic acid amides and the like in the text even though it does not describe a synthesis method of them.
However, in the case that a fine pattern with a pitch less than 200 nm is to be formed, problems regarding size difference (dependency on density) between an isolated pattern and a dense pattern having different optical contrast from each other and LER (line edge roughness) which shows roughness of a pattern have become larger. As a further finer pattern rule is being requested, remedial measures for avoiding deterioration of resolution and LER in addition to providing an excellent performance in sensitivity, an adhesion property with a substrate and etching resistance have been needed.
In such a movement, for improving line edge roughness, there is attempted an incorporation of acroyloxyphenyldiphenylsulfonium salt monomers in a base resin (see Japanese Patent Laid-Open (kokai) No. 20005-84365). However, in the above attempt, the monomers are bonded to the polymer at their cation side and therefore, a sulfonic acid generated by high energy beam irradiation is not different from a sulfonic acid generated from a conventional photoacid generator and they are not sufficient for solving the above problems. Moreover, there is also disclosed a sulfonium salt in which anion side such as polystyrene sulfonic acid is incorporated in a main chain of a polymer for improving sensitivity and resist pattern profile (see Japanese Patent No. 3613491). However, each generated acids is a derivative of an arene sulfonic acid or an alkyl sulfonic acid and has low acid strength and therefore, the generated acids are insufficient to scissor acid labile groups, especially acid labile groups of ArF chemically amplified resist compositions. Japanese Patent Laid-Open (kokai) No. 2006-178317 discloses many polymers having a partially fluorinated sulfonic acid anion as a polymerizable unit and resist compositions. Moreover, Japanese Patent Laid-Open (kokai) No. 2007-197718 describes three kinds of anions specifically, but these anions are expected to be highly hydrolyzable and low in stability because they are carboxylic acid ester of a strong acid. In addition, solubility of obtained copolymers in a resist solvent is not sufficient. Furthermore, Japanese Patent Laid-Open (kokai) No. 2008-133448 also discloses a sulfonium salt having a partially fluorinated alkanesulfonic acid anion as a polymerizable unit, but resolution is insufficient even though LER is improved a little, and therefore, it has been impossible to satisfy both of these parameters.
As an exposure technology after ArF lithography, electron beam (EB) lithography, F2 lithography, EUV (extreme ultraviolet beam) lithography, X ray lithography and the like are drawing an attention as a promising technology. However, a sulfonic acid generated during exposure vaporizes due to a necessity of performing exposure under vacuum (under reduced pressure) and thereby there are a problem that a good pattern profile cannot be obtained and a possibility that the vaporized sulfonic acid damages an exposure apparatus. In addition, in EB and EUV lithography, it has been desired that resist compositions which can further suppress an impact of acid diffusion and provide a high resolution are developed for keeping up with a further pattern miniaturization in recent years.
The foregoing is essential also as a mask patterning process especially in an electron beam lithography, which is drawing an attention as an ultra-miniaturization process technology with 0.1 μm or less dimension. Deterioration of the pattern profile on a mask blanks becomes a serious problem in processing of the photomask blanks, because it also causes a pattern collapse as the integrated circuit pattern progresses toward further miniaturization in recent years.